How to update iOS version on iPhone 6?

For mobile device system software, read this document: Update your iPhone or iPad - Apple Support

For apps on a mobile device open the App Store app, then Updates, then select the person icon in the upper right corner. Click update All, or scroll down past the first screen to select individual apps.

Your iPhone 6 can only be updated to iOS 12.5.7

The instructions have already been posted above.